Angiogenesis plays important roles in tumor growth and metastasis (Folkman, 1971, 1995; Hanahan and Folkman, 1996; Hanahan, 1997), and numerous molecules regulate angiogenesis (Ingber and Folkman, 1989; Folkman and D'Amore, 1996; Risau, 1997). Angiopoietins are ligands of Tie-2 receptor kinase which is primarily expressed by endothelial cells (ECs) and their precursors (Sato et al., 1993; Schnurch and Risau, 1993; Dumont et al., 1994). Three Tie-2 ligands have been identified (Davis et al., 1996; Maisonpierre et al., 1997; Valenzuela et al., 1999): angiopoietin-1, -2, and -3 (Ang-1, -2, and -3). Angiopoietin has a similar protein structure, which consists of a signal peptide, an amino terminal coiled-coil domain, a short linker peptide region, and a carboxyl terminal fibrinogen homology domain (FHD). The coiled-coil domain is responsible for dimerization/mulimerization of angiopoietin, while the FHD binds to Tie-2 receptor (Maisonpierre et al., 1997; Procopio et al., 1999; Valenzuela et al., 1999).
Studies have shown that Ang-1-Tie-2 pathway plays an essential role at the late stage of vascular development. Targeted disruption of Ang-1 or Tie-2 or overexpression of Ang-2 results in embryonic death with similar vascular defects. These mice displayed normal vascular growth factor (VEGF)-dependent early vascular development, but with profound defects in remodeling, organization, and stabilization of the primitive vasculature (Dumont et al., 1994; Sato et al., 1995; Suri et al., 1996; Maisonpierre et al., 1997).
At least under some circumstances, Ang-2 and Ang-3 are considered as naturally occurring antagonists of Tie-2 (Maisonpierre et al., 1997; Valenzuela et al., 1999). Ang-1 activates Tie-2 receptor by inducing tyrosine phosphorylation of Tie-2 and promotes recruitment of pericytes and smooth muscle cells. Thus, Ang-1 plays an important role in establishing and maintaining vascular integrity. As antagonists of Tie-2, Ang-2 and Ang-3 are believed to compete with Ang-1 for binding of Tie-2 and to block Tie-2 phosphorylation induced by Ang-1 (Maisonpierre et al., 1997; Valenzuela et al., 1999; Yancopoulos et al., 2000). It is well documented that angiogenesis is regulated by balanced activities of pro- and anti-angiogenic factors (Hanahan and Folkman, 1996; Risau, 1997; Hanahan, 1997). The existence of these antagonists underscores importance for precise regulation of Tie-2 activity.
It has been shown that VEGF and angiopoietin-Tie-2 pathways are independent and essential for tumor angiogenesis (Lin et al., 1997, 1998; Siemeister et al., 1999). Recent data showed that Ang-1 and Ang-2 are expressed by tumor cells and involved in tumor angiogenesis (Stratmann et al., 1998; Yu and Stamenkovic, 2001; Ahmad et al., 2001; Etoh et al., 2001; Hawighorst et al., 2002). However, the function of Ang-3 in general and the role of Ang-3 in tumor angiogenesis and metastasis have not been established.
Angiogenesis has also been tied to other vascular disorders such as, diabetes and arthritis, for which there is a need for the development of better treatments.
Thus, there is a need to identify the function and role of Ang-3 in tumor angiogenesis and metastasis. There is also a need for methods of identifying modulators of Ang-3 activity. There is also a need in the art for methods of treating individuals with cancer and other diseases related to uncontrolled angiogenesis and caused by lacking of angiogenesis and poor vascular endothelial cell health. The present invention fulfills these needs and others.